[Coley]

I would look to seriously narrow down the points where they are likely to get in and then work to fully block those entry points.
Again noting that a mouse can get through a hole these size of a dime.
I would guess it would be possibly through the kick panel vents or up along/under the dash somewhere.
Even if you have to remove a heater vent or something along those lines.....it would be worth it.
All vermin are terrible carriers of disease and stuff so you don't want to be around that too much if you can avoid it....plus that smell of mice in a vehicle is a real bummer.
Spend the time and make up some parts to put in there and you will beat this problem...although that won't keep them from enjoying under the hood...but maybe a few mousetraps, etc can help limit that.

Coley